In conventional ceramic processing, e.g. slip casting, the ceramic slurries usually have to have a particle load as high as possible to obtain an intermediate body with a high green density. A high green density is desired and needed to enable the production of dense sintered ceramics.
Powder-based additive manufacturing technologies where the low packing density of the powder bed results in a highly porous 3D object, typically does not result in a high density ceramic without the addition of large amounts of pressure during heat treatment, making the realization of dense complex three dimension shapes challenging. Typically this method leads to densities of less than 95% of the theoretical density of the ceramic material.
The processing of slurries based on ceramic filled photopolymers with stereolithography has shown promise due to its ability to serve as a green body in the production of relatively dense ceramic articles with three dimensional architecture. Meanwhile, there are efforts trying to also use additive manufacturing technologies (like SLA technology), which are mainly used for processing polymers, for the production of ceramic articles.
However, a high particle load in a slurry can be disadvantageous for the rheological properties needed to process the slurry in an additive manufacturing technique.
On the other hand, reducing the particle load in the slurry will result in an article with a low green density which cannot be sintered to full density without cracks.
It was also observed that the intermediate body resulting from the additive manufacturing process is often not self-supporting.
U.S. Pat. No. 7,927,538 B2 (Moszner et al.) describes light-curing slips for the stereolithographic preparation of dental ceramics. The slip comprises a polyreactive binder, photoinitiator, surface-modified ceramic particles and a chain transfer agent. The viscosity of the slip lies in the range of 200 mPa*s to 2,000 Pa*s (23° C.).
U.S. Pat. No. 6,283,997M (Garg et al.) relates to a process for producing a ceramic composite bone implant having a porous network from a photocurable polymer with a high volume percent of ceramic composition. For producing the photocurable ceramic composition, alumina or hydroxyapatite having a particle size in the range of 0.05 to 10 μm (microns) is suggested.
U.S. Pat. No. 8,003,040 B2 (El-Siblani) relates to a process for producing a 3-dim object by solidifying layers with electromagnetic radiation of synergistic stimulation in a pattern.
US 2007/0072762 (Neil et al.) describes a method of making ceramic discharge vessels for a lamp application using stereolithography. The ceramic-resin mixture used for this method contains a photocurable acrylate resin and ceramic powders like aluminum oxide, aluminum oxynitride, yttrium aluminum garnet and aluminum nitride powders having a mean grain size in the range of d50=0.6 μm. The viscosity of the mixture is in a range of 200 to 25,000 mPa·s.
U.S. Pat. No. 6,955,776 (Feenstra) relates to a method for making a dental element by using a powder-based 3D printing technique. The powder can be used in dry form or in dispersed in form (slurry). The powder can be ceramic material or a metal. The ceramic material is preferably selected from SiO2, Al2O3, K2O, Na2O, CaO, Ba2O, CrO2, TiO2, BaO, CeO2, La2O3, MgO, ZnO and Li2O. The powder used in the example has median particle size of d50: 0.5 to 0.7 μm.
US 2003/0222366 A1 (Stangel et al.) describes a dental restoration production in which a digitized optical impression of a dental restoration site is captured using an intra-oral camera. The captured optical impression is converted into a data file suitable for computer-assisted production using stereolithography. The ceramic-containing material should have a viscosity in the range of 200 to 3.5 million centipoise (mPa*s). The mean particle size of the ceramic material should be from 0.05 to 5 μm.
U.S. Pat. No. 8,133,831 (Laubersheimer et al.) describes a slip for the preparation of dental ceramics by a hot-melt inkjet printing process. The slip contains ceramic particles, a radically polymerizable monomer and a wax. Ceramic particles based on Al2O3 or ZrO2 should have a particle size of 50 to 500 nm (nanometers).
Similarly, US2012/308837 A1 (Schlechtriemen et al.) describes a process for the generative preparation of shaped ceramic bodies by 3D inkjet printing using different kinds of ceramic slips. The viscosity of the slips is said to be above 200 mPa*s at room temperature.
US 2014/0183799 A1 (Fischer et al.) deals with light-curing ceramic slips for the stereolithographic preparation of high-strength ceramics using a slip based on a radically polymerizable binder, polymerization initiator, filler and a certain acidic monomer comprising a radically polymerizable group. For Y-TZP zirconium dioxide, a particle size in the range of 50 to 3500 nm is said to be preferred. The rheological properties of the slip are said to be in a range from 0.02 to 20,000 Pa*s (23° C.).
U.S. Pat. No. 8,329,296 B2 (Apel et al.) relates to primary particles of oxide-ceramic material having a primary particle size in the range of 10 to 1,000 nm which are coated with a chromophoric component. The particles may be provided as a suspension comprising a polyreactive binder, an organic solvent and additives. The suspension is said to have a viscosity from 200 to 2,000 Pa*s (23° C.).
WO 01/13815 A1 (Feenstra) describes a method for making a dental element by a 3-dim printing technique. As curable material preferably a nanomeric material consisting of nanomeric inorganic solid particles having polymerizable organic groups at their surface is used. After the printing process, the dental element is typically subject to a thermal post-treatment between 60 and 150° C. to complete curing. Instead thereof, or supplemental thereof, a thermal densification is accomplished wherein the dental element is heated to a temperature of at least 250° C. However, the compositions described in the references above have deficiencies.
Often, using a slurry or slip with ceramic particles greater than 50 nm in diameter is suggested. Not only are the slurry properties typically not suitable to produce highly accurate ceramic articles, but the larger particle sizes, even when closely packed, still limit the percentage of theoretical density possible, limiting the final material properties including mechanical as well as optical performance. Thus, there is a need for an improved additive manufacturing process.
There is also a need for high strength, translucent printed article, preferably a high strength, translucent printed zirconia article.